A nurse is reviewing the medical record of a newly admitted client who has major depressive disorder. Which of the following findings should the nurse identify as a risk factor for this condition?

A. The client has a serotonin deficiency: A deficiency in serotonin, a neurotransmitter that regulates mood, sleep, and cognition, is a well-established biological risk factor for major depressive disorder. Reduced serotonin activity can contribute to persistent low mood and other depressive symptoms. B. The client has acute bronchitis: While acute illness can temporarily affect mood, acute bronchitis is not a recognized risk factor for developing major depressive disorder. Its effects are usually transient and resolve with treatment of the illness. C. The client has an elevated calcium level: Hypercalcemia can cause physical symptoms such as fatigue or confusion but is not directly linked to the pathogenesis of major depressive disorder. D. The client is an only child: Birth order or being an only child does not constitute a risk factor for depression. Genetic, biochemical, and psychosocial factors play a more significant role in the development of the disorder.